The wireless power transfer (WPT) using magnetic resonant concept was proposed by Nikola Tesla more than 100 years ago. Until recently, with the development of power electronics technology, it is realized that a wireless power transfer system can be implemented economically enough to have a commercial value. Several companies have already developed products which can transfer power with acceptable power level and efficiency through a certain air gap.
In a wireless power transfer system, the energy is transferred through the mutual inductance of the transmit coil and receive coil, while the leakage inductance does not have a direct contribution to the active power transfer. Because of the large gap between the transmit coil and receive coil, the coupling coefficient between the two coils is small, typically in the range of 5% to 30% depending on the distance, alignment, and size of the coils. This causes the wireless power transfer systems to have a large leakage inductance but a small mutual inductance. To increase the coupling, the coil design, without a doubt, is important. Meanwhile, the compensation circuit which is used to cancel the leakage inductance, is also of great importance. Usually, capacitors, which can be lumped or parasitic, are added to form a resonant circuit, which is known as the magnetic resonant method.
Different compensation topologies have been proposed and implemented to tune the two coils working at a resonant frequency in a wide range of applications. There are four basic topologies depending on how the compensation capacitors are added to the transmit and receive coils, namely, series-series (SS), series-parallel (SP), parallel-series (PS), and parallel-parallel (PP) topologies. Some other novel topologies have also been proposed in the literature. For example, a dual-topology is realized by switching between a parallel compensated and a series compensated secondary side to realize both constant current mode and constant voltage mode. Moreover, the transmit and receive coils need to be connected to the power electronics converters. To achieve high efficiency for the complete wireless power transfer system, some additional topologies have been proposed. In another example, a LCL converter is formed by adding LC compensation network between the converter and the transmit coil. There are two advantages for the LCL converter when the system works at the resonant frequency. First, the inverter only supplies the active power required by the load; second, the current in the primary side coil is independent of the load condition. In yet another example, a LC compensation network at both primary and secondary sides is proposed for bidirectional power transfer. The design of a LCL converter usually requires the same value for the two inductors.
One of the uniqueness of wireless power transfer systems is the high spatial freedom of the coils. This means the air gap variation and misalignment of the transmit and receive coils are inevitable. Usually, the system parameters and resonant frequency of the primary and secondary resonant tanks change when the coupling condition changes. With traditional compensation topologies, in order to achieve high efficiency, a tuning method is needed to maintain the resonance when the air gap changes or misalignment happens. There are two main methods, namely frequency control and impedance matching. Phase-locked loop techniques are able to tune the operating frequency to track the resonant frequency which will change due to the variation of gap length, misalignment and tolerance variation of the tuning components. Alternatively, impedance matching can be applied. Either method is difficult to implement in practice due to the uncertainty of the variations of the system. The overall system efficiency has also been constrained.
In this disclosure, a double-sided LCC compensation topology and its parameter design method is proposed. The topology consists of one inductor and two capacitors at both the primary and secondary sides. With the proposed method, the resonant frequency of the compensated coils is independent of the coupling coefficient and the load condition. The wireless power transfer system can work at a constant frequency, which eases the control and narrows the occupation of frequency bandwidth. Nearly unit power factors can be achieved for both the primary side and the secondary side converters in the whole range of coupling and load conditions, thus high efficiency for the overall system is easily achieved. A parameter tuning method is also proposed and analyzed to achieve ZVS operation for the MOSFET-based inverter. The proposed method is more attractive in an environment where the coupling coefficient keeps changing, like the electric vehicle charging application. Also, due to its symmetrical structure, the proposed method can be used in a bi-directional WPT system. Simulation and experimental results verified analysis and validity of the proposed compensation network and the tuning method. A prototype with 7.7 kW output power for electric vehicles was built, and 96% efficiency from DC power source to battery load is achieved.
This section provides background information related to the present disclosure which is not necessarily prior art.